1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an in-plane switching (IPS) mode LCD device in which liquid crystal molecules are aligned at multiple angles in one unit pixel region to improve response speed without reducing an aperture ratio.
2. Discussion of the Related Art
Recently, flat panel displays have been studied actively including LCD devices. An LCD device varies optical anisotropy of a liquid crystal having fluidity and optical properties by applying electric field to the liquid crystal. The LCD device is widely used owing to features and advantages including being lightweight, a large-sized screen, high resolution, and low power consumption in comparison with a cathode ray tube (CRT).
The LCD device has various operating modes depending on the characteristics of the liquid crystal and the electrode structures. Example modes of the LCD device include a twisted nematic (TN) mode LCD device, a multi-domain mode LCD device, an optically compensated birefringence (OCB) mode LCD device, a vertical alignment (VA) mode LCD device, and an IPS mode LCD device.
In the TN mode LCD device, liquid crystal directors are arranged at a twisted angle of 90° and voltages are applied thereto to control the liquid crystal directors. In the multi-domain mode LCD device, one pixel is divided into a plurality of domains and main viewing angles of the respective domains vary from one another to obtain a wide viewing angle for the device as a whole. In the OCB mode LCD device, a compensation film is attached to an outer surface of a substrate to compensate a phase variation of light depending on a progress direction of the light. In the VA mode LCD device, liquid crystal molecules are vertically arranged on an alignment film using a negative liquid crystal and a vertical alignment film. In the IPS mode LCD device, two electrodes are formed on one substrate and liquid crystal directors are twisted in parallel with an alignment film.
Among them, the IPS mode LCD device includes a color filter array substrate and a thin film transistor array substrate arranged to oppose each other by interposing a liquid crystal layer therebetween. The color filter array substrate is provided with a black matrix layer to prevent light leakage and R/G/B color filter layers formed on the black matrix layer to display colors. The thin film transistor array substrate is provided with gate and data lines for defining unit pixels, thin film transistors formed at each crossing points between the respective gate and data lines, and common and pixel electrodes alternately arranged to generate transverse electric field.
Hereinafter, related art IPS mode LCD devices will be described with reference to the accompanying drawings. FIG. 1 is a plane view illustrating a unit pixel of a first related art IPS mode LCD device, FIG. 2 is a plane view illustrating a unit pixel of a second related art IPS mode LCD device, and FIG. 3 is a graph illustrating voltage-transmittance of the related art devices.
As shown in FIG. 1, a thin film transistor array substrate of the first related art IPS mode LCD device includes gate lines 12 and data lines 15 perpendicularly crossing the gate lines 12 to define unit pixels, thin film transistors 20, and common lines 25. The unit pixel includes, a plurality of common electrodes 24 and a plurality of pixel electrodes 17. The thin film transistors 20 are formed in unit pixels and serve as switching elements. The common lines 25 are formed in parallel with the gate lines 12. The common electrodes 24 branch from the common lines 25 in a single body and arranged along the gate lines 12 in the unit pixels. The pixel electrodes 17 are alternately formed between and are parallel with the common electrodes 24.
One unit pixel region is divided into a plurality of blocks 30 by the pixel and common electrodes 17 and 24. The common lines 25 and the common electrodes 24 are supplied with signals Vcom from the contour of an active region. Each pixel electrode 17 is connected to a drain electrode 15b of each thin film transistor 20 to receive a pixel signal so that liquid crystal molecules in the blocks 30 are rearranged by the transverse electric field formed between the common electrode 24 and the pixel electrode 17.
In FIG. 1, the pixel electrode 17 and the common electrode 24 are arranged along the gate line 12 at an angle α of 10° relative to the gate line (0°). The liquid crystal molecules are initially aligned in a direction of the gate line 12 and then rearranged by the transverse electric field in a direction perpendicular to the pixel electrode 17 and the common electrode 24 to determine transmittance of light. In a general IPS mode device, since the maximum transmittance angle of the liquid crystal molecules is 45° with respect to a rubbing direction, the initially aligned liquid crystal molecules are rotated by 45°-α, i.e., 35° when a driving voltage is applied thereto.
In the first related art IPS mode LCD device as shown in FIG. 1, the unit pixel region is divided into sixteen blocks 30.
However, a problem is present relating to response speed of the first related art IPS mode LCD device. Namely, a response time of the first related art device is slow. To solve this problem, as shown in FIG. 2, pixel electrodes 117 and common electrodes 124 are arranged along gate lines 112 at an angle β of 20° relative to the gate lines 112 (0°) in the second related art device.
In the second related art device, liquid crystal molecules are initially aligned along the gate lines 112 and then rearranged by the transverse electric field in a direction perpendicular to the pixel electrodes 117 and the common electrodes 124. As noted above, the maximum transmittance angle of the liquid crystal molecules is 45° around a rubbing direction. Thus, in this instance, the initially aligned liquid crystal molecules are rotated by an angle of 45°-β, i.e., 25° when the driving voltage is applied thereto in the second related art device. Since rotating the liquid crystal molecules for 25° occurs more quickly than rotating for 35°, the response speed of the second related art IPS mode LCD device is improved over the first related art device.
On the other hand, since each pixel electrode 117 and each common electrode 124 are arranged with a bent angle of 20° relative to the gate line 112 in the second related art device instead of 10° as in the first related art device, the number of blocks 130 of the pixel electrode and the common electrode is reduced to fourteen, which is less than the number of blocks 30 in the first related art device.
In other words, if the arrangement angle of the pixel electrode and the common electrode is increased, the number of electrodes formed in the unit pixel region having a fixed size is correspondingly decreased in the related art devices, thereby reducing the number of the blocks. When the number of blocks, i.e. the number of pixel and common electrodes, is reduced, the luminance is also reduced. Therefore, a tradeoff in the related art devices is that an improvement in the response time comes at a cost of reduction in luminance.
In other words, the related art IPS mode LCD devices have the following problems.
In the IPS mode LCD device in which the common electrode and the pixel electrode are arranged along the gate line, if the common electrode and the pixel electrode are arranged with the bent angle of 10° relative to the gate line, luminance of the device can be improved since the number of blocks can be increased. However, since the rotational angle of the liquid crystal molecules is large, the response speed is decreased.
On the other hand, if the common electrode and the pixel electrode are arranged with the bent angle of 20° relative to the gate line, the response speed of the liquid crystal molecules may be increased. However, the number of the blocks formed in the unit pixel region of the fixed size is reduced due to the increased angle of the electrodes, thereby reducing an opening region of the device.
Another problem is present in related art devices of FIGS. 1 and 2. As shown in FIG. 3 which illustrate voltage-transmittance characteristics of the related art IPS mode LCD devices, the saturation voltages Vsat1 and Vsat2 are both small in width.